Electrolytic lesions of the paraventricular nucleus (PVN) of the hypothalamus were found to disrupt the initiation but not the maintenance of maternal behavior in primiparous Sprague-Dawley rats. Following lesions performed on Day 15 of gestation, measures of maternal behavior (grouping, crouching, and nest building), pup retrieval, and pup weight gain were all impaired, but only if the lesion included the most rostral and medial aspects of the PVN. Lesions sparing these regions and sham surgery were generally ineffective. In a separate experiment, PVN lesions performed on Day 4 postpartum had relatively little effect on maternal behavior. As females lesioned prepartum showed changes in open-field behavior as well as maternal behavior, the PVN may be important for modulating responses not only to pups but to several kinds of novel stimuli. These results may also further implicate oxytocin, which is synthesized in the PVN, in the initiation of maternal behavior.